On the question of limiting the adjustments, I think this is practical mainly from the perspective of someone with a limited number of frame designs in mind. Sachs makes a lot of cyclo cross bikes I believe, and he makes them all with the same BB drop, and he won't compromise. So a person who wanted to make a few cross bikes, might model that input and decide they could get away with a rear unit that fixed chainstay length, and BB drop, probably even seat post angle if they knew their measurements for sure, and that is often the same number anyway. That would leave one moveable part the headtube positioning. I came up with a number of ways to make jigs of this type very simply, though I never did them because I am the opposite type of builder. I build only for myself, and every bike is likely to be different. A cargo bike, a touring bike, a folder, an MTB, a tandem. Just changing wheel diameter or even tire type might lead me to change the BB drop. The simplified jig is useless to me.
Since jigs aren't necesarry or arguably even desireable, and bad jigs aren't particularly useful, I think for many small shops that are not commercial in orientation it is hard to make a case for jigs. The main reason people build them is because they want them. It seems like the main specialist tool for a frame builder. Like the main tool for a cabinetmaker is the workbench.
I have been down the welded jig path, and have been pretty disappointed. The welded jig is very difficult to make accurate, because essentially the problems are the same as framebuilding; it is tough to super heat metal and keep it aligned. The added challenge is that most weld fab steel is grossly inaccurate. If I were thinking of composite frames where the parts don't move under heat, or even require tight tolerances in assembly, epoxy and cloth bridge gaps, it would be pretty hard to argue me out of plywood jigs. It is easy to get into the accuracy range of milled metals, with cheap tools that cut very quickly. I can take a 2x4 edge and make it straight within a thou every 2 feet in a matter of minutes even seconds. Much faster compared to the time it takes to machine anything, that would take me many hours and I don't have the gear to do it on large pieces.
My way to have my cake and eat it too is to use milled metal parts scavenged from machines. They are true and made of stable materials. They are cheap and already pre-machined. Using these parts is still taking a lot of time because even the small amount of machining left over is costly and time consuming.
Last edited by NoReg; 02-08-09 at 01:57 AM.